This work reports an experimental study on the temperature dependence of the structural parameters of LiCrO${}_2$ (LCO) and Li${}_{0.99}$Cu${}_{0.01}$CrO${}_2$ (LCCO) by using a synchrotron x-ray diffraction technique. A significant magnetoelastic coupling is revealed by the anomalies observed in lattice parameters at the magnetic and electric phase transitions, apparent as steplike features in both Cr-O and Li-O bond lengths, as well as in O-Cr-O bond angles. Magnetic, dielectric, and electric polarization measurements reveal the antiferromagnetic and antiferroelectric (AFE) ordering at 119 and 61 K, respectively, for LCCO. Interestingly, a fairly large uncompensated spontaneous electric polarization appears for LCCO in contrast to nearly compensated polarization value for LCO below the AFE ordering. This is correlated to the structurally driven enhancement ($\sim$4 times) of the interlayer Cr-O-Li/Cu-O-Cr superexchange interaction for LCCO. We argue that strong magnetoelastic coupling holds the key for the observed uncompensated spontaneous electric polarization in LCCO.

We present a microscopic theory of spin torque due to diffusive spin currents induced by spin accumulation. The obtained expression is a natural extension of the existing one due to “local” spin currents associated with ordinary electric currents, and is the reciprocal of the spin motive force which induces charge accumulation as studied recently [J. Shibata and H. Kohno, Phys. Rev. B 84, 184408 (2011)]. The result is applied to a domain wall motion in a nonlocal spin injection system, and the torque and force due to diffusive spin current are evaluated.

Scaling of the anomalous Hall conductivity to longitudinal conductivity ${\sigma }_{\mathrm{AH}}\propto {\sigma }_{xx}^2$ has been observed in the dirty regime of two-dimensional weak and strong localization regions in ultrathin, polycrystalline, chemically disordered, ferromagnetic FePt films. The relationship between electron transport and temperature reveals a quantitatively insignificant Coulomb interaction in these films, while the temperature dependent anomalous Hall conductivity experiences quantum correction from electron localization. At the onset of this correction, the low-temperature anomalous Hall resistivity begins to be saturated when the thickness of the FePt film is reduced, and the corresponding Hall conductivity scaling exponent becomes 2, which is above the recent unified theory of 1.6 (${\sigma }_{\mathrm{AH}}\propto {\sigma }_{xx}^{1.6}$). Our results strongly suggest that the correction of the electron localization modulates the scaling exponent of the anomalous Hall effect.

Using first-principles calculations, we propose a two-dimensional diluted magnetic semiconductor: monolayer MoS${}_2$ doped by transition metals. Doping of transition metal atoms from the IIIB to VIB groups results in nonmagnetic states, since the number of valence electrons is smaller or equal to that of Mo. Doping of atoms from the VIIB to IIB groups becomes energetically less and less favorable. Magnetism is observed for Mn, Fe, Co, Zn, Cd, and Hg doping, while for the other dopants from these groups it is suppressed by Jahn-Teller distortions. Analysis of the binding energies and magnetic properties indicates that (Mo,$X$)S${}_2$ ($X=\text{Mn}$, Fe, Co, and Zn) are promising systems to explore two-dimensional diluted magnetic semiconductors.

The theory of the stationary Josephson tunnel current $I_c$ was devised for junctions involving superconductors partially gapped by biaxial or unidirectional charge density waves (CDWs) and possessing a superconducting order parameter of $d$-wave symmetry. Specific calculations were carried out for symmetric junctions between two identical CDW superconducting electrodes and nonsymmetric ones composed of a CDW superconductor and a conventional isotropic $s$-wave superconductor. Two kinds of superconducting pairing symmetries were studied, namely, that appropriate to cuprates ($d_{x^2-y^2}$) and the $d_{xy}$ one. The corresponding calculations were also carried out for the extended $s$-wave symmetry of the superconducting order parameter. Allowances were made for the directionality of tunneling. In all the cases studied, the dependences of $I_c$ on the angle $\gamma$ between the chosen crystal direction and the normal to the junction plane were found to be significantly influenced by CDWs. It was shown in particular that the $d$-wave driven periodicity of $I_c\left(\gamma \right)$ in the CDW-free case is transformed into double-period beatings depending on the parameters of the system. The results of calculations testify that the orientation-dependent patterns $I_c\left(\gamma \right)$ measured for CDW superconductors allow the CDW configuration (unidirectional or checkerboard) and the symmetry of superconducting order parameter to be determined. The predicted effects can be used to indirectly reveal CDWs in underdoped cuprates where pseudogaps are observed. The break-junction technique is an appropriate tool for this purpose.

When energy is deposited in a thin-film cryogenic detector, such as from the absorption of an x-ray, an important feature that determines the energy resolution is the amount of athermal energy that can be lost to the heat bath prior to the elementary excitation systems coming into thermal equilibrium. This form of energy loss will be position dependent and therefore can limit the detector energy resolution. An understanding of the physical processes that occur when elementary excitations are generated in metal films on dielectric substrates is important for the design and optimization of a number of different types of low-temperature detectors. We have measured the total energy loss in one relatively simple geometry that allows us to study these processes and compare measurements with calculation based upon a model for the various different processes. We have modeled the athermal phonon energy loss in this device by finding an evolving phonon distribution function that solves the system of kinetic equations for the interacting system of electrons and phonons. Using measurements of device parameters such as the Debye energy and the thermal diffusivity we have calculated the expected energy loss from this detector geometry, and also the position-dependent variation of this loss. We have also calculated the predicted impact on measured spectral lineshapes and have shown that they agree well with measurements. In addition, we have tested this model by using it to predict the performance of a number of other types of detector with different geometries, where good agreement is also found.

We study the second quantized or guiding center description of the torus Laughlin state. Our main focus is the change of the guiding center degrees of freedom with the torus geometry, which we show to be generated by a two-body operator. We demonstrate that this operator can be used to evolve the full torus Laughlin state at given modular parameter $\tau$ from its simple (Slater-determinant) thin torus limit, thus giving rise to a new presentation of the torus Laughlin state in terms of its “root partition” and an exponential of a two-body operator. This operator therefore generates, in particular, the adiabatic evolution between Laughlin states on regular tori and the quasi-one-dimensional thin torus limit. We make contact with the recently introduced notion of a “Hall viscosity” for fractional quantum Hall states to which our two-body operator is naturally related and which serves as a demonstration of our method to generate the Laughlin state on the torus.

The incorporation of bismuth (Bi) in GaAs results in a large reduction of the band gap energy ($E_g$) accompanied with a large increase in the spin-orbit splitting energy (${△}_{SO}$), leading to the condition that ${△}_{SO}>E_g$, which is anticipated to reduce hot-hole producing Auger recombination losses whereby the energy and momentum of a recombining electron-hole pair are given to a second hole which is excited into the spin-orbit band. We theoretically investigate the electronic structure of experimentally grown GaBi${}_x$As${}_{1-x}$ samples on (100) GaAs substrates by directly comparing our data with room temperature photomodulated reflectance (PR) measurements. Our atomistic theoretical calculations, in agreement with the PR measurements, confirm that $E_g$ is equal to ${△}_{SO}$ for $\mathit{\text{x}}\approx$ 9$\%$. We then theoretically probe the inhomogeneous broadening of the interband transition energies as a function of the alloy disorder. The broadening associated with spin-split-off transitions arises from conventional alloy effects, while the behavior of the heavy-hole transitions can be well described using a valence band-anticrossing model. We show that for the samples containing 8.5$\%$ and 10.4$\%$ Bi the difficulty in identifying a clear light-hole-related transition energy from the measured PR data is due to the significant broadening of the host matrix light-hole states as a result of the presence of a large number of Bi resonant states in the same energy range and disorder in the alloy. We further provide quantitative estimates of the impact of supercell size and the assumed random distribution of Bi atoms on the interband transition energies in GaBi${}_x$As${}_{1-x}$. Our calculations support a type-I band alignment at the GaBi${}_x$As${}_{1-x}$/GaAs interface, consistent with recent experimental findings.

I consider the nonequilibrium dc transport of electrons through a quantum system with a thermoelectric response. This system may be any nanostructure or molecule modeled by the nonlinear scattering theory, which includes Hartree-like electrostatic interactions exactly, and certain dynamic interaction effects (decoherence and relaxation) phenomenologically. This theory is believed to be a reasonable model when single-electron charging effects are negligible. I derive three fundamental bounds for such quantum systems coupled to multiple macroscopic reservoirs, one of which may be superconducting. These bounds affect nonlinear heating (such as Joule heating), work and entropy production. Two bounds correspond to the first law and second law of thermodynamics in classical physics. The third bound is quantum (wavelength dependent), and is as important as the thermodynamic ones in limiting the capabilities of mesoscopic heat engines and refrigerators. The quantum bound also leads to Nernst's unattainability principle that the quantum system cannot cool a reservoir to absolute zero in a finite time, although it can get exponentially close.

The electronic structure of the Rh-based CuRh${}_{0.9}$Mg${}_{0.1}$O${}_2$ oxide thermoelectric compound has been studied with a multitechnique approach consisting of photoemission, x-ray absorption, and x-ray emission spectroscopies. The data indicate that the region of the valence band in the proximity of the Fermi level is dominated by Rh-derived states. These findings outline the importance of the electronic structure of the Rh ions for the large thermoelectric power in CuRh${}_{0.9}$Mg${}_{0.1}$O${}_2$ at high temperature.

We theoretically investigate the local density of states (LDOS) probed by an STM tip of ferromagnetic metals hosting a single adatom and a subsurface impurity. We model the system via the two-impurity Anderson Hamiltonian. By using the equation of motion with the relevant Green's functions, we derive analytical expressions for the LDOS of two host types: a surface and a quantum wire. The LDOS reveals Friedel-like oscillations and Fano interference as a function of the STM tip position. These oscillations strongly depend on the host dimension. Interestingly, we find that the spin-dependent Fermi wave numbers of the hosts give rise to spin-polarized quantum beats in the LDOS. Although the LDOS for the metallic surface shows a damped beating pattern, it exhibits the opposite behavior in the quantum wire. Due to this absence of damping, the wire operates as a spatially resolved spin filter with a high efficiency.

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in the presence of spin polarized two-dimensional Dirac fermions. We show that an externally applied spin polarization along the $z$ axis mediates an anisotropic interaction which corresponds to an $XXZ$ model interaction between two magnetic moments. For undoped graphene, while the $x$ part of interaction keeps its constant ferromagnetic sign, its $z$ part oscillates with the distance $R$ of magnetic impurities. A finite doping causes both parts of the interaction to oscillate with $R$. We explore a beating pattern of oscillations of the RKKY interaction along armchair and zigzag lattice directions, which occurs for some certain values of the chemical potential. The two characteristic periods of the beating are determined by inverse of the difference and the sum of the chemical potential and the spin polarization.

Correlated electronic and structural distortions in individual carbon nanotubes induced by photon excitation are observed for the first time by using a unique in situ transmission electron microscope configuration. By combining sub-100-meV resolution electron energy-loss spectroscopy with in situ photon excitation of the specimen, we are able to measure carbon nanotube expansions coupled to electronic transitions in their density of states as a direct consequence of the photon excitation/irradiation. Laser-induced effects are recorded in electron energy-loss spectra, which show reversible changes in the carbon unoccupied conduction band with 2$p$ character, as well as in the carbon plasmon spectral band, for both individual single-wall and individual multiwall carbon nanotubes. Furthermore, for single-walled carbon nanotubes, we see significant changes in the density of states close to the Fermi energy. The observed changes are discussed in the context of increased interatomic spacing, resulting from photon-induced thermal expansion and exciton screening enhancement due to photocarriers. These are the first measurements of photon-induced correlated structural and electronic changes at the atomic level in any nanoscale material. This new capability and technique should enable a new understanding of photon-induced structural, chemical, and electronic reactions across a range of nanoscale systems.

Using ab initio molecular dynamics simulations, we calculate the equation of state of iron in the solid phase for both the hcp and bcc structures as well as the high-pressure melting curve up to 15 Mbars. We first find that the melting temperature increases up to 11 000 K at the highest pressures investigated following a semiempirical melting law over the entire pressure domain. We also investigate the stability of the bcc phase of iron beyond Earth's core conditions (3 Mbars) and find that the temperature at which the bcc phase is mechanically stabilized increases with density. Finally, we provide simple fits of these results for convenient use in the modeling of Earth-like exoplanets up to ten Earth masses, which requires accurate knowledge of the properties of iron up to 15 Mbars.

We present a semiclassical treatment of a fermionic impurity coupled to Bogolyubov modes of a Bose-Einstein condensate (BEC). In the lowest-order approximation we find a full solution of an initial value problem, which turns out to behave differently in the sub- and supersonic regimes. While in the former case no impurity deceleration is observed, in the latter case non-Markovian dissipation effects kick in, resulting in a slowing down of the fermion. Although this scenario is compatible with the one offered by an elementary field-theoretical picture at weak coupling, the details of the dynamics turn out to be completely different. Fluctuation effects can be taken into account by expanding around the classical path, which leads to a natural cutoff scale for the momentum integrals of the order of the inverse polaron radius. As an application we calculate the drag force which is exerted by the BEC on the impurity moving with constant velocity $v$. Contrary to the perturbative result, according to which the drag force is $\sim v^4$, it turns out to be proportional to $1/v^2$ in the semiclassical regime.

Effects of high pressure exceeding 10 GPa on spin-driven ferroelectricity were investigated for a multiferroic, triangular-lattice antiferromagnet (TLA), CuCrO${}_2$. For this purpose, we developed a system which enables us to measure ferroelectric polarization under a pressure of 10 GPa by using a diamond anvil cell. We found that the magnetic transition temperature accompanying the ferroelectric one in CuCrO${}_2$ was remarkably enhanced by applying pressure. The result is simply explained by considering the pressure-induced enhancement of inter- and/or intralayer magnetic interaction due to the compression of the lattice. In addition, the coercive electric field for the polarization reversal was also increased with increasing pressure, while the amplitude of the ferroelectric polarization was steeply suppressed at around 8 GPa. A possible origin of the observed pressure effects on the ferroelectric property in the multiferroic TLA is discussed in terms of a ferroelectric-antiferroelectric transition and structural domain rearrangement by uniaxial stress.

The multipole expansion of magnetostatic interaction energy between nonuniformly magnetized ferromagnetic particles is developed and applied for calculation of the gyrotropic frequencies of the coupled vortex state dots. A pair of coupled dots in a vortex state is considered as a model system, and then the calculations are extended to lateral arrays of the interacting vortex state dots [one- and two-dimensional (2D) square and hexagonal lattices]. The odd rank multipole moments (dipolar, octupolar, etc.) play the main role in the interdot magnetostatic interaction. The vortex collective frequencies and group velocities are calculated within the pole-free and rigid vortex models and compared with the experimental observations of the dynamics by broadband ferromagnetic resonance and x-ray imaging. The cases of different vortex core polarizations are considered, and their strong influence on the excitation spectra is shown. Frequency bandwidths of the vortex gyrotropic modes for dense 2D square and hexagonal arrays can exceed 1/3 of the eigenfrequency of an isolated dot. The group velocity for transferring the total microwave magnetic energy from one dot to another is found to be proportional to the dot thickness and inversely proportional to the squared lattice period.

Electronic structure of newly synthesized single crystals of calcium iron arsenide doped with sodium with $T_{\mathrm{c}}$ ranging from 33 to 14 K has been determined by angle-resolved photoemission spectroscopy (ARPES). The measured band dispersion is in general agreement with theoretical calculations, nonetheless implies absence of Fermi-surface nesting at an antiferromagnetic vector. A clearly developing below $T_{\mathrm{c}}$ strongly band-dependant superconducting gap has been revealed for samples with various doping levels. The BCS ratio for optimal doping, $2\Delta /k_{\mathrm{B}}{T}_{\mathrm{c}}=5.5$, is substantially smaller than the numbers observed for related compounds.

We report a characterization of surfaces of the dichalcogenide TaSe${}_2$ using scanning tunneling microscopy and spectroscopy at 150 mK. When the top layer has the $2H$ structure and the layer immediately below the $1T$ structure, we find a singular spatial dependence of the tunneling conductance below 1 K, changing from a zero-bias peak on top of Se atoms to a gap in between Se atoms. The zero-bias peak is additionally modulated by the commensurate $3a_0\times 3a_0$ charge-density wave of $2H$-TaSe${}_2$. Multilayers of $2H$-TaSe${}_2$ show a spatially homogeneous superconducting gap with a critical temperature also of 1 K. We discuss possible origins for the peculiar tunneling conductance in single layers.